Early-out *before* looking up an enabled feature if being removed from a window.

This fixes a crash (caught by ASAN) when the feature list gets torn down
before the window containing the download shelf.

Bug: 806901
Change-Id: I00ddc55d183f2b29bf23ad3304b7c700f747894f
Reviewed-on: https://chromium-review.googlesource.com/899735Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534452}

chrome/browser/ui/cocoa/download/download_shelf_view_cocoa.mm

@@ -129,10 +129,10 @@ constexpr CGFloat kMDShelfHeight = 44;
 }
 
 - (void)adjustHeightForDivider {
-  if (!base::FeatureList::IsEnabled(features::kMacMaterialDesignDownloadShelf))
-    return;
   if (!self.window)
     return;
+  if (!base::FeatureList::IsEnabled(features::kMacMaterialDesignDownloadShelf))
+    return;
   [self stopAnimation];
   if (NSHeight([self bounds])) {
     [self setHeight:kMDShelfHeight + [self cr_lineWidth]];